This invention relates generally to game balls, and more particularly to a game ball, such as a golf ball, having an ionomeric cover.
Before the development of ionomers, balata was the preferred material for golf ball covers. Polyethylene also was proposed for use as a golf ball cover material but was generally deemed highly inferior to balata in imparting playability and durability characteristics to the ball due to its brittleness and high hardness, and thus never became a commercially successful golf ball cover material.
Balata golf ball covers have now been replaced to a great extent by ionomeric cover materials. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E. I. DuPont deNemours and Company (see U.S. Pat. No. 4,884,814) under the trademark Surlyn(copyright) and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks Escor(copyright) and the tradename xe2x80x9clotekxe2x80x9d, have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the overall durability necessary for repetitive play.
Ionomeric resins are generally ionic copolymers or terpolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Optionally, an acrylate can also be present. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc., for golf ball cover construction over balata.
In this regard, the metal ions serve as crosslinking agents, as they are ionically bonded to carboxylic acid groups in adjacent copolymer chains. However, instead of having thermally irreversible covalent bonding, ionomers have thermolabile crosslinking in which metal ions become part of the chemical structure of the ionomer upon crosslinking, and these crosslinks are reversible. Consequently, at high temperatures ionomers are subject to distortion or degradation.
Furthermore, the advantages gained through the use of ionomer resins in golf ball cover formulations in increased durability have been offset to some degree by the decreases produced in playability. This is because although the ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the xe2x80x9chardxe2x80x9d ionomeric resins, but also the playability (i.e. xe2x80x9cspinxe2x80x9d) characteristics previously associated with the xe2x80x9csoftxe2x80x9d balata covers, properties which are still desired by the more skilled golfer.
In various attempts to produce such an ideal golf ball, the golfing industry has blended the hard ionomeric resins with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been dissatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various xe2x80x9chard-soft ionomeric blendsxe2x80x9d, i.e. mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. However, until the development of the specific blend combination set forth in U.S. Pat. No. 4,884,814, directed to low modulus golf ball cover compositions, these balls were not particularly commercially viable. In this regard, although the balls produced using the hard-soft ionomer blends exhibited enhanced playability characteristics, they lacked the durability needed for continuous play.
Moreover, while there are numerous advantages to the use of ionomers in making golf ball covers, one drawback of conventional golf balls with ionomeric covers are that the covers are prone to softening at temperatures of approximately 50xc2x0 C. or above. As a result, the ionomeric covers (and in particular, soft ionomeric covers) may lose their dimple pattern or develop flat spots if exposed to high temperatures.
Accordingly, it would be useful to develop a golf ball with an ionomeric cover which is highly resistant to high temperature distortion or degradation without sacrificing the properties of distance, durability and/or playability.
Furthermore, it would be useful to improve the heat resistance of soft ionomeric golf ball covers without substantially hardening the covers. This would result in the production of a soft ionomer covered golf ball having the distance, durability and/or playability characteristics desired while also being resistant to degradation at high temperatures.
There are numerous advantages to use of ionomers in making golf ball covers. On the other hand, one drawback of conventional golf balls with soft ionomeric covers are that the covers are prone to scuffing and cutting, particularly when hit with irons which have sharp grooves. It would be useful to develop a golf ball with a soft ionomeric cover which is highly resistant to cutting and scuffing by sharp-grooved clubs.
An object of the invention is to provide an ionomeric game ball cover having improved scuff resistance and/or cut resistance.
Another object of the invention is to provide a method for imparting improved scuff resistance and/or cut resistance to a game ball cover.
Yet another object of the invention is to provide a golf ball with a soft cover which is well-suited for use with golf club irons having sharp grooves.
Yet another object of the invention is to provide a method of forming a golf ball with a soft cover which has excellent scuff resistance and/or cut resistance.
Another object of the invention is to provide a method for improving the heat and/or melt resistance of a golf ball cover. The invention is directed to any type of ionomer covered golf ball including unitary, wound, two-piece, three-piece and multi-layered golf balls.
Yet another object of the invention is to provide a golf ball with a soft ionomer cover which is well-suited for repetitive play and exhibits improved heat and/or melt resistance when subject to high temperatures, i.e. 50xc2x0 C. or more.
Another object of the invention is to provide a golf ball with a very soft ionomeric cover having enhanced heat stability and/or improved heat and melt resistance. These golf balls also exhibit the feel and playability characteristics that highly-skilled golfers prefer. This enables the soft covered golf balls to exhibit enhanced dimple retention during prolonged exposure to high temperatures.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
The invention in a preferred form is a game ball having a cover comprising an ionomer resin. The ionomer resin includes a copolymer of an olefin and an xcex1, xcex2-ethylenically unsaturated carboxylic acid which is about 10-100% neutralized with metal ions. The copolymer has a sufficient degree of covalent crosslinking to impart to the cover improved resistance to at least one of scuffing and cutting. In a particularly preferred form of the invention, the game ball is a golf ball. Preferably, the game ball has a dimpled surface. The covalent crosslinking preferably comprises irradiation-induced covalent crosslinking.
In a preferred form of the invention, the degree of covalent crosslinking is appropriate to impart to the cover a Shore D hardness which is no more than about 10% greater, and more preferably no more than about 5% greater, than the Shore D hardness of a cover having an identical composition but which does not include a substantial degree of covalent crosslinking. Preferably, the copolymer includes an acrylate.
Another preferred form of the invention is a method of treating a game ball. The method comprises the steps of obtaining a game ball having a cover comprising an ionomer, and irradiating the ionomer in the cover under conditions appropriate to covalently crosslink the ionomer in order to increase the resistance of the cover to at least one of scuffing and cutting with substantially impairing other playability characteristics of the ball. In a particularly preferred form of the invention, the game ball is a golf ball. Preferably, the game ball has a dimpled surface.
According to the preferred method of the invention, the game ball is subjected to electron beam treatment at a dosage of at least 2 megarads. The game ball cover preferably is irradiated prior to application of a top coat over a cover. The method of the invention preferably further includes the step of applying a top coat over the cover before or after irradiation.
More particularly, the outer cover is a blend of one or more ionomer copolymers and/or terpolymers and one or more ionomers having a high Vicat softening temperature. Preferably, the high Vicat softening temperature ionomer also has a high melt temperature. More preferably, the difference between the high melt temperature and the Vicat softening temperature of the high melt ionomer is minimized. It has been found that such high melt ionomers act as good heat stability modifiers for ionomer covers, and in particular, for soft ionomer covered golf balls.
Along these lines, the Vicat softening temperature of the high melt ionomer incorporated into the present invention is 74xc2x0 C. or more, preferably 80xc2x0 C. or more, and most preferably, 84xc2x0 C. or more. The melt temperature of the high melt ionomer is 96xc2x0 C. or more, preferably 98xc2x0 C. or more, and most preferably, 100xc2x0 C. or more. Furthermore, the high melt ionomer utilized in the invention exhibits a difference in melt temperature and Vicat temperature of 25xc2x0 C., more preferably 19xc2x0 C. or less, most preferably 17xc2x0 C. or less.
Additionally, the high melt ionomer of the present invention can also be utilized to formulate that inner cover layer or mantle of a multi-layer golf ball. Accordingly, one or more of the ionomer layers of a multi-layer golf ball can exhibit high heat stability.
The dimpled cover golf ball of the present invention preferably has a Shore D hardness cover of 63 or less, preferably 55 or less, and most preferably 50 or less.
Additionally, the present invention is directed to a golf ball comprising a core in combination with the improved covers that includes a particular combination of polybutadiene rubbers, and a cover disposed about the core which includes a specific combination of ionomer resins. The polybutadiene rubbers used in the particular combination include a first polybutadiene rubber that is obtained utilizing a cobalt catalyst and which exhibits a Mooney viscosity in the range of from about 70 to about 83. The combination of polybutadiene rubbers also includes a second polybutadiene rubber that is obtained utilizing a neodymium series catalyst or a blend of neodymium catalyzed polybutadienes and which exhibits a Mooney viscosity of from about 30 to about 70.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements exemplified in the following detailed disclosure.